feat: add option to log warnings about redirected links

[major0]

closes #617

[cjmayo]

Can be useful to know about redirects.

Needs a test. We do have checker/test_http_redirect.py. Probably some of the tests in there can be reused, returning warning now instead of info when this is enabled?

My main comment, is that log-redirects is not the right name. The logging is a result of making a redirect a warning instead of info - and logging could still be avoided with --no-warnings.

And. of course, documentation in the man pages.

[cjmayo]

Maybe we actually make it a warning with a tag name (http-????, as https://linkchecker.github.io/linkchecker/man/linkcheckerrc.html#warnings) then it could be disabled if not required with ignorewarnings https://linkchecker.github.io/linkchecker/man/linkcheckerrc.html#url-checking-results?

As things are at the moment if it is a problem for anyone they can just stay on 10.2.1 until ready to update their config.

[ssbarnea]

I really hope to see this essential feature in. One of the most common problems is about URLs that get outdated and replaced by redirects. Now we cannot detect these redirects. What makes it worse is that sometimes these redirects are not sending to right content and instead to the external site root.

[cjmayo]

I think a new http- warning is the way to go (we really want to avoid more options where possible). Slightly more complex example of adding a warning: d6936ce - no need for a new method should just be replacing the add_info identified in this PR with add_warning.

[cjmayo]

Warning implemented in #750